Optical grade wafers are used for the most demanding optical applications such as high speed modulators and polarization-controlling devices. Optical material is low strain with no inclusions. SAW grade wafers are used in non-optical applications and are the least demanding in terms of impurities and crystalline perfections. Refractive grade wafers are used for lower demanding optical applications such as Wollaston prisms for DVD heads and blur-filters for digital still cameras. This material is ideal for uses where strain is tolerable, but inclusions are not.